I think it's both. I think its a vast trove of villainy, but also a ninja paradox.
Like, we've seen several stories about the Phantom King where he ruins Clark's whole day by himself, yet when Clark was in the Zone recently, he dealt with all the inmates at once and lived to tell the tale.
I am definitely all about getting more mileage out of the Zone inmates, but it might be better to deal with them one (or two) at a time, instead of trying to involve everyone who ever showed up between the Silver Age and now.